1. Field of the Invention
The present invention relates to apparatus for monitoring and ensuring the proper connection of three phases of electric power to a load, and more particularly to apparatus which automatically detects improper connection and alters the connection for proper phase relationship between the supply and the load connections.
2. Description of the Related Art
Long-haul trucks often have a cargo box that is refrigerated in order to transport food and other loads which require refrigeration. When the truck is moving, a refrigeration unit, that is powered by electricity from an alternator driven by an engine, maintains the cargo box interior at the proper temperature. However when the truck is parked, such as at a distribution center or rest area, the refrigeration unit is commonly connected to electrical utility lines at that site in order to conserve fuel and eliminate engine exhaust emissions. That connection is accomplished by a cable that has a plug which engages a receptacle to which the source of electrical power the refrigeration unit has a connection plug to which the electrical utility lines are attached.
Many refrigeration units operate on three-phase electrical power and it is critical that the three prongs of the refrigeration unit plug be properly connected to the three phases (designated A, B and C) of alternating currents in the electrical utility lines. Specifically, phase angle of the phase A voltage must lead to phase angle of the phase B voltage by 120 degrees and the phase angle of the phase B voltage must in turn lead to the phase angle of the phase C voltage by 120 degrees. This provides a phase sequence commonly referred to as ABC. Therefore, the voltage each prong of plug is intended to receive either the A, B or C phase so that the electricity is supplied with the proper phase relationships. If any of the two phases are swapped in the connection of the plug to the electrical utility lines, the phasing is referred to as ACB. In that situation the angle of the phase A voltage lags the phase angle of the phase B voltage by 120 degrees and the phase angle of the phase B voltage lags the phase angle of the phase C voltage by 120 degrees. This results in the electrical voltage being incorrectly applied to the refrigeration unit, causing improper operation and possible damage to the motors and other components. For example, a direction that a three phase motor rotates depends upon the phase sequence.
Therefore, the proper electrical connection requires that the prongs of the refrigeration unit plug and the contacts of the utility receptacle be wired in the same manner or the proper phase relationship will not exist. To prevent improper connection, the plugs and receptacles are designed to mate in only one orientation and there is a defined standard for connecting the phase lines to the terminals of plug and the receptacle. However, it is not uncommon that a receptacle for the electrical utility lines is improperly wired. As a consequence, occasionally a truck's refrigeration equipment is damaged by an improper electrical connection.
To prevent such damage, electrical phase controllers have been used to interface the power cable to the refrigeration unit. These controllers either disconnected the equipment from the electrical connector when an improper phase sequence is found to exist or had set of contactors which reversed the connections so that the proper phase sequence is applied to the refrigeration equipment. The prior electrical phase controllers devices had one contactor that applied the ABC lines of the cable to the ABC input terminals of the refrigeration unit and another contactor that swapped the improperly connected cable lines so that the proper current phases were applied to the ABC input terminals. A phase monitor detected the actual phase sequence and activated the appropriate contactor of the controller.
It is also important to detect the absence of one of the three phases as the equipment is designed to operate correctly when all three phases are present. When one of the phases is not present, larger stresses are placed on the remaining phase and this can result in overheating of motors and other electrical equipment. Apparatus also has been provided to disconnect supply lines from electrical equipment when a missing phase is detected.
A further problem exists when the supply voltage is significantly less than or greater than the nominal specified voltage for the equipment. During such conditions devices, such as motors, do not operate efficiently and can often overheat. Here too monitors have been provided in electrical equipment to disconnect the supply line connections during significant under voltage situations.
Monitoring apparatus for phase sequence, a missing phase, and under/over voltages perform by sensing the voltages on each of the three phases. Therefore, a particular monitoring device heretofore was specifically designed for a particular utility line voltage of the country in which the apparatus was to be used. In the United States for example, the three phases are typically at 220 volts, whereas in other countries the phase voltage may be 400 volts, or more. The frequency of the alternating electricity also is either 50 HZ or 60 Hz depending upon the country. Thus separate monitoring circuits were designed and manufactured based on the country in which that equipment was to be utilized. Providing different models of electrical monitors for operation at different phase voltages and frequencies complicated the manufacture, storage and distribution of the devices.
Therefore it is desirable to provide an electrical supply monitor that has electrical circuitry capable of properly functioning over a wide range of input voltages.